Electrode kinetics reversible region

Fignre 2.41 indicates that the net peak current is a parabolic function of the electrode kinetic parameter. This is illnstrated in Fig. 2.43. With respect to the electrochemical reversibility of the electrode reaction, approximately three distinct regions can be identified. The reaction is totally irreversible for log(ca) < — 2 and reversible for log(ft)) > 2. Within this interval, the reaction is qnasireversible. The parabolic dependence of the net peak cnrrent on the logarithm of the kinetic parameter asso-... 

A quasireversible electrode reaction is controlled by the film thickness parameter A, and additionally by the electrode kinetic parameter k. The definition and physical meaning of the latter parameter is the same as for quasireversible reaction under semi-infinite diffusion conditions (Sect. 2.1.2). Like for a reversible reaction, the dimensionless net peak current depends sigmoidally on the logarithm of the thickness parameter. The typical region of restricted diffusion depends slightly on K. For instance, for log( If) = -0.6, the reaction is under restricted diffusion condition within the interval log(A) < 0.2, whereas for log(if) = 0.6, the corresponding interval is log(A) <0.4. 

Define and explain the following terms for electrode kinetics irreversible, quasi-reversible, linear region, and reversible. (Bockris)... 

Figure 2. Examples of numerical solutions for the cathodic current distribution on a plate electrode immersed in a cell with the counter electrode at the bottom. Three cases are compared (a) (/ column) completely reversible kinetics (primary distribution) (b) center) irttermedrate kinetics (Ub 0.2) (c) (right column) irreversible kinetics (Wa 10). The top row provides a comparison of the current distribution or the deposit profile on the cathode (cross-hatched region). The center row provides the current distribution along the electrode ( stretched ). The bottom row provides the corresponding poterrtial distributions. It is evident that the current distribution uniformity increases as the electrode kinetics become more passivated (Cell-Design software simulations ).

It follows from the figures and also from an analysis of Eq. (6.40) that in the particular case being discussed, electrode operation is almost purely diffusion controlled at all potentials when flij>5. By convention, reactions of this type are called reversible (reactions thermodynamically in equilibrium). When this ratio is decreased, a region of mixed control arises at low current densities. When the ratio falls below 0.05, we are in a region of almost purely kinetic control. In the case of reactions for which the ratio has values of less than 0.02, the kinetic region is not restricted to low values of polarization but extends partly to high values of polarization. By convention, such reactions are called irreversible. We must remember... 

The concentration profiles are very sensitive to the kinetics of the electrode reaction. In this context, the determination of the diffusion layer thickness is of great importance in the study of non-reversible charge transfer processes. This magnitude can be defined as the thickness of the region adjacent to the electrode surface where the concentration of electro-active species differs from its bulk value, and it can be accurately calculated from the concentration profiles. In the previous chapter, the extensively used concept of Nemst diffusion layer (8), defined as the distance at which the linear concentration profile (obtained from the straight line tangent to the concentration profile curve at the electrode surface) takes its bulk value, has been explained. In this chapter, we will refer to it as linear diffusion layer since the term Nemst can be misunderstood when non-reversible processes... 

Concerning the determination of kinetic parameters of the voltammograms of quasi-reversible and irreversible electrode processes, Fig. 3.10b shows the existence of different linear zones in a similar way to that observed for planar electrodes (see Fig. 3.6). For practical purposes, it is helpful to use spherical microelectrodes, for which a broader linear region is obtained under steady-state conditions, since the process behaves as more irreversible as the radius decreases. For fully irreversible charge transfers, Eq. (3.74) simplifies to... 

In the region where no reverse peak is observed, the pure kinetic zone, it can be shown that the chemical reaction has the effect of of shifting the cathodic peak potential positive of the value for the reversible electron transfer. This is because the coupled chemical reaction reduces the concentration of Rat the surface from the value it would have had for a simple electron transfer reaction. The electrode reaction therefore has to work harder to maintain Nernstian equilibrium... 

Thus, in the Nernstian regime, a plot of / vs. / - will be linear, and useful information about the parameters n and Dr can be obtained from its slope for the electrode process of interest. (Double potential step experiments similarly afford information about the reverse process, reduction.) Likewise a plot of it vs, (Fig. 20.7c) yields kinetics information for a non-Nernstian process. The horizontal region at large values of it - corresponds to the Cottrell regime, whereas the short-time data are... 

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